The attached drawing marked “Prior Art” illustrates in schematic longitudinal cross-section a typical jet engine EN configuration. A number of rotary or, in this illustration, blade sections A, B, C, D, are provided at different functional stages within the engine EN. Operation of a jet engine EN is relatively well known and clearly as illustrated involved airflows (shown by arrowheads) in order to create propulsion through combustion derived by operation of the engine EN. One of the determinant factors with respect to engine operation efficiency is the amount of leakage of airflows about tips of the blades in comparison with flows through and across the blades of the engine stages A, B, C, D. A particular area of leakage is that about the distal spinning tip end of each blade. However, it is necessary to provide some clearance between the distal or tip end of each blade and a casing or cowling within which the blades rotate. It is of particular importance that the gap should be predictable rather than absolutely minimised. As the blades rotate they define a tip edge profile which is spaced by the gap from the casing.
It will be understood that it is generally an objective to achieve as small tip clearance as possible throughout an engines operational cycle, but particularly during normal engine operation, such as at cruise. The engine operates in the cruise state for the longest periods of time and so the advantages of well regulated tip clearance will therefore be beneficial.